Dynamics of N-OH bond dissociation in cyclopentanone and cyclohexanone oxime at 193 nm: laser-induced fluorescence detection of nascent OH (v', J')

Cyclohexanone oxime (CHO) and cyclopentanone oxime (CPO) in the vapor phase undergo N-OH bond scission upon excitation at 193 nm to produce OH, which was detected state selectively employing laser-induced fluorescence. The measured energy distribution between fragments for both oximes suggests that in CHO the OH produced is mostly vibrationally cold, with moderate rotational excitation, whereas in CPO the OH fragment is also formed in v' = 1 (~2%). The rotational population of OH (v' = 0, J') from CHO is characterized by a rotational temperature of 1440 ± 80 K, whereas the rotational populations of OH (v' = 0, J') and OH (v' = 1, J') from CPO are characterized by temperatures of 1360 ± 90 K and 930 ± 170 K, respectively. A high fraction of the available energy is partitioned to the relative translation of the fragments with f(T) values of 0.25 and 0.22 for CHO and CPO, respectively. In the case of CHO, the Λ-doublet states of the nascent OH radical are populated almost equally in lower rotational quantum levels N', with a preference for Π(+) (A') states for higher N'. However, there is no preference for either of the two spin orbit states Π(3/2) and Π(1/2) of OH. The nascent OH product in CPO is equally distributed in both Λ-doublet states of Π(+) (A') and Π(-) (A') for all N', but has a preference for the Π(3/2) spin orbit state. Experimental work in combination with theoretical calculations suggests that both CHO and CPO molecules at 193 nm are excited to the S(2) state, which undergoes nonradiative relaxation to the T(2) state. Subsequently, molecules undergo the N-OH bond dissociation from the T(2) state with an exit barrier to produce OH (v', J').     

Photodissociation dynamics of phenol

The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H(2)O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H(2)O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 相似文献   

Dynamics of OH radical generation in laser-induced photodissociation of tetrahydropyran at 193 nm

Tetrahydropyran (THP) undergoes photodissociation on excitation with ArF laser at 193 nm, generating OH radical as one of the transient photoproducts. Laser-induced fluorescence technique is used to detect the nascent OH radical and measure its energy state distribution. The OH radical is formed mostly in the ground vibrational level (v"=0), with low rotational excitation. The rotational distribution of OH (v"=0,J) is characterized by a temperature of 433+/-31 K, corresponding to a rotational energy of 0.86+/-0.06 kcalmol. Two Lambda-doublet levels, 2Pi+(A') and 2Pi-(A"), and the two spin-orbit states, the 2Pi(3/2) and 2Pi(1/2), of OH are populated statistically for all rotational levels. The relative translational energy associated with the photoproducts in the OH channel is calculated to be 21.9+/-3.2 kcal mol(-1), from the Doppler-broadened linewidth, giving an ft value of approximately 43%, and most of the remaining 57% of the available energy is distributed in the internal modes of the other photofragment, C5H9. The observed distribution of the available energy is explained well, using a hybrid model of energy partitioning, with an exit barrier of 40 kcal mol(-1). The potential-energy surface of the reaction channel was mapped by ab initio molecular-orbital calculations. Based on experimental and theoretical results, a mechanism for OH formation is proposed. Electronically excited THP relaxes to the ground electronic state, and from there, a sequence of reactions takes place, generating OH. The proposed mechanism first involves C-O bond scission, followed by a 1,3 H atom migration to O atom, and finally, the C-OH bond cleavage giving OH.     

State-to-state correlated study of CD3I photodissociation at 266 and 304 nm

High-resolution photofragment translational spectroscopy is used in this work to measure the translational and internal energy distributions in the CD3 and iodine fragments produced from the photodissociation of CD3I at 266 and 304 nm. Channel selected detection, via resonantly enhanced multiphoton ionization, combined with one-dimensional core sampling provides detailed information about vibrational state distributions of the CD3 fragments. The vibrational state distributions of CD3 fragments in the I*(2P12) channel have a propensity of nu2 ' umbrella bending mode with a maximum at nu2 ' = 1 for 266 nm photodissociation. For I*(2P12) channel at 304 nm photodissociation, vibrational state distributions of CD3 fragment have a maximum in the vibrational ground state. For the I(2P32) channel (1Q1 <-- 3Q(0+)), nu2 ' umbrella bending vibrational distribution is measured as the predominant vibrational mode but has a much broader distribution when compared to that of the I* channel. The vibrational state distributions of the CD3 fragment produced from the perpendicular transition, i.e., 3Q1, which was determined at 304 nm photodissociation, has a maximum at nu2 ' = 1. The curve crossing possibility between the 1Q1 and 3Q(0+) adiabatic potentials is determined as 0.19 for 266 and 0.85 for 304 nm. The trend in reaction dynamics in 266 and 304 nm photodissociation of CD3I is compared with theoretical calculations. A bond dissociation energy D0(C-I) = 56.60+/-0.5 kcal/mol was derived by applying laws of energy conservation.     

Molecular elimination of methyl formate in photolysis at 234 nm: roaming vs. transition state-type mechanism

Ion imaging coupled with (2 + 1) resonance-enhanced multiphoton ionization (REMPI) technique is employed to probe CO(v″ = 0) fragments at different rotational levels following photodissociation of methyl formate (HCOOCH(3)) at 234 nm. When the rotational level, J″(CO), is larger than 24, only a broad translational energy distribution extending beyond 70 kcal mol(-1) with an average energy of about 23 kcal mol(-1) appears. The dissociation process is initiated on the energetic ground state HCOOCH(3) that surpasses a tight transition state along the reaction coordinate prior to breaking into CO + CH(3)OH. This molecular dissociation pathway accounts for the CO fragment with larger rotational energy and large translational energy. As J″(CO) decreases, a bimodal distribution arises with one broad component and the other sharp component carrying the average energy of only 1-2 kcal mol(-1). The branching ratio of the sharp component increases with a decrease of J″(CO); (7.3 ± 0.6)% is reached as the image is probed at J″(CO) = 10. The production of a sharp component is ascribed to a roaming mechanism that has the following features: a small total translational energy, a low rotational energy partitioning in CO, but a large internal energy in the CH(3)OH co-product. The internal energy deposition in the fragments shows distinct difference from those via the conventional transition state.     

OH fragment from benzoic acid monomer photolysis: threshold and product state distribution

Photodissociation dynamics of benzoic acid monomer (BAM) at different ultraviolet excitation wavelengths (280-295 nm) has been investigated. The nascent OH product state distributions were measured using the laser-induced fluorescence (LIF) technique. The rotational state distributions, the Lambda-doublet-state ratio, and spin-orbit state distributions of the OH fragment were also measured at 280-294 nm. The OH fragments are vibrationally cold, and their rotational state distributions are peaked at J' = 3.5 at each photolysis wavelength. No LIF signal of OH fragments was observed at 295 nm. The photodissociation threshold is determined to be 102.5-103.9 kcal/mol for OH channel. The dissociative state and mechanism have been discussed for OH produced from the photodissociation of BAM.     

Distributions of angular anisotropy and kinetic energy of products from the photodissociation of methanol at 157 nm

We investigated distributions of angular-anisotropy parameter beta and kinetic energy of fragments after photodissociation of methanol using time-of-flight (TOF) mass spectrometry. Fragments, in particular CH(3)O and CO, were successfully detected using tunable radiation from a synchrotron for photoionization. Following O-H bond fission, a CH(3)O fragment with internal energy greater than 104 kJ mol(-1) dissociates to CH(2)O+H. Elimination of two H(2) accompanies formation of CO. The beta value of hydroxyl hydrogen is -0.26 whereas that of methyl hydrogen is zero. H(2) has two distinct components in TOF spectra; these rapid and slow components have beta values -0.30 and -0.18, respectively. The CH(3)+OH dissociation exhibits a highly anisotropic angular distribution with beta= -0.75. The beta values of fragments from CD(3)OH photolysis are addressed. From measurements of angular-anisotropy parameters of various fragments, we surmise that the transition dipole moment mu is almost perpendicular to the C-O-H plane and that n-3p(x) (2 (1)A") is the major photoexcited state at 157 nm.     

正丙醇和异丙醇的紫外光解动力学

利用高里德堡态氢原子飞行时间(HRTOF)探测技术,研究了正丙醇和异丙醇的紫外光解动力学过程.在193.3 nm光辐射下,O-H键快速断裂过程构成主要的氢原子生成通道.伴随O-H键的碎裂,相当大的一部分能量转换成氢原子及其相应碎片的平动能(正丙醇〈fv〉=0.76; 异丙醇〈fv〉=0.78).氢原子碎片具有各向异性的角度分布;其角分布异向因子β分别为-0.79(正丙醇)和-0.77(异丙醇).研究结果表明,吸收1个193.3 nm光子后,丙醇分子跃迁到一个寿命很短的电子激发态;沿着O-H反应坐标,该激发态势能面是排斥的,因而O-H键快速断裂.此外,还得到了丙醇的O-H键离解能: (432±2)kJ/mol(正丙醇)和(433±2)kJ/mol(异丙醇).     

Dissociation dynamics of thiolactic acid at 193 nm: detection of the nascent OH product by laser-induced fluorescence

Electronically excited thiolactic acid (2-mercaptopropionic acid), H(3)C-CH(SH)-COOH, undergoes the C-OH bond cleavage on excitation to the S(2) state at 193 nm, generating the primary product OH (v,J), which is detected by laser-induced fluorescence technique in a collisionless condition of flow system. The partitioning of the available energy between vibrational, rotational, and translational degrees of freedom of nascent photofragments is obtained from relative intensities of ro-vibronic lines in laser-induced fluorescence spectrum of OH, and their Doppler profiles. The rotational population of OH (v(")=0) is characterized by rotational temperature of 408+/-25 K. OH is produced in a vibrationally cold state, i.e., mostly in v(")=0. The average translational energy of OH (v(")=0,J(")) is found to be 21.5+/-2.0 kcal/mol, which implies 25.6 kcal/mol of energy in relative translation of photoproducts corresponding to the f(t) value of approximately 0.6. The observed high translational energy is due to the presence of a barrier in the exit channel, implying that the C-OH bond scission takes place on an electronically excited potential energy surface. The observed partitioning of the available energy between various degrees of the photofragments is theoretically modeled, and the hybrid model, with 26.0 kcal/mol of barrier in the exit channel, is found to explain the measured data quite well. The experimental results are also supported with ab initio molecular orbital calculations for both the ground and the excited electronic states. Time-dependent density functional theory is used to understand the nature of various electronic transitions connecting the lower excited states. Potential energy curves as a function of the C-OH bond length of thiolactic acid suggest distinct exit barriers in the S(1), T(1), and T(2) states. But, we could locate the transition state structure for OH formation in the S(1) state alone. Thus, although thiolactic acid is excited to the S(2) state at 193 nm, it undergoes internal conversion to S(1) where it dissociates to yield OH. In addition to the OH channel from excited electronic states, we studied theoretically all probable dissociation channels occurring on the ground electronic state of thiolactic acid.     

Photodissociation dynamics of formyl fluoride (HFCO) at 193 nm: branching ratios and distributions of kinetic energy

Following photodissociation of formyl fluoride (HFCO) at 193 nm, we detected products with fragmentation translational spectroscopy utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Among three primary dissociation channels observed in this work, the F-elimination channel HFCO-->HCO+F dominates, with a branching ratio approximately 0.66 and an average release of kinetic energy approximately 55 kJ mol(-1); about 17% of HCO further decomposes to H+CO. The H-elimination channel HFCO-->FCO+H has a branching ratio approximately 0.28 and an average release of kinetic energy approximately 99 kJ mol(-1); about 21% of FCO further decomposes to F+CO. The F-elimination channel likely proceeds via the S1 surface whereas the H-elimination channel proceeds via the T1 surface; both channels exhibit moderate barriers for dissociation. The molecular HF-elimination channel HFCO-->HF+CO, correlating with the ground electronic surface, has a branching ratio of only approximately 0.06; the average translational release of 93 kJ mol(-1), approximately 15% of available energy, implies that the fragments are highly internally excited. Detailed mechanisms of photodissociation are discussed.     

Effects of torsion on O-H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.     

Molecular elimination in photolysis of o- and p-fluorotoluene at 193 nm: Internal energy of HF determined with time-resolved Fourier transform spectroscopy

Following the photodissociation of o-fluorotoluene [o-C(6)H(4)(CH(3))F] at 193 nm, rotationally resolved emission spectra of HF(1< or =v< or =4) in the spectral region of 2800-4000 cm(-1) are detected with a step-scan Fourier transform spectrometer. HF(v< or =4) shows nearly Boltzmann-type rotational distributions corresponding to a temperature approximately 1080 K; a short extrapolation from data in the period of 0.5-4.5 mus leads to a nascent rotational temperature of 1130+/-100 K with an average rotational energy of 9+/-2 kJ mol(-1). The observed vibrational distribution of (v=1):(v=2):(v=3)=67.6: 23.2: 9.2 corresponds to a vibrational temperature of 5330+/-270 K. An average vibrational energy of 25+/-(3) (12) kJ mol(-1) is derived based on the observed population of HF(1< or =v< or =3) and estimates of the population of HF (v=0 and 4) by extrapolation. Experiments performed on p-fluorotoluene [p-C(6)H(4)(CH(3))F] yielded similar results with an average rotational energy of 9+/-2 kJ mol(-1) and vibrational energy of 26+/-(3) (12) kJ mol(-1) for HF. The observed distributions of internal energy of HF in both cases are consistent with that expected for four-center elimination. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF. An observed vibrational energy of HF produced from fluorotoluene slightly smaller than that from fluorobenzene might indicate the involvement of seven-membered-ring isomers upon photolysis.     

Molecular elimination in photolysis of fluorobenzene at 193 nm: internal energy of HF determined with time-resolved Fourier-transform spectroscopy

Following photodissociation of fluorobenzene (C(6)H(5)F) at 193 nm, rotationally resolved emission spectra of HF(1相似文献   

Photodissociation dynamics of fluorobenzene (C6H5F) at 157 and 193 nm: Branching ratios and distributions of kinetic energy

Following photodissociation of fluorobenzene (C6H5F) at 193 and 157 nm, we detected the products with fragmentation-translational spectroscopy by utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Between two primary dissociation channels observed upon irradiation at 193 (157) nm, the HF-elimination channel C6H5F --> HF + C6H4 dominates, with a branching ratio of 0.94+/-0.02 (0.61+/-0.05) and an average release of kinetic energy of 103 (108) kJ mol(-1); the H-elimination channel C6H5F --> H + C6H4F has a branching ratio of 0.06+/-0.02 (0.39+/-0.05) and an average release of kinetic energy of 18.6 (26.8) kJ mol(-1). Photofragments H, HF, C6H4, and C6H4F produced via the one-photon process have nearly isotropic angular distributions. Both the HF-elimination and the H-elimination channels likely proceed via the ground-state electronic surface following internal conversion of C6H5F; these channels exhibit small fractions of kinetic energy release from the available energy, indicating that the molecular fragments are highly internally excited. We also determined the ionization energy of C6H4F to be 8.6+/-0.2 eV.     

Gas-phase photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy

By using time-resolved Fourier-transform infrared emission spectroscopy, the fragments of HCN(v = 1, 2) and CO(v = 1-3) are detected in one-photon dissociation of acetyl cyanide (CH(3)COCN) at 308 nm. The S(1)(A(")), (1)(n(O), π(?) (CO)) state at 308 nm has a radiative lifetime of 0.46 ± 0.01 μs, long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of Ar collision-induced internal conversion is estimated to be (1-7) × 10(-12) cm(3) molecule(-1) s(-1). The measurements of O(2) dependence exclude the production possibility of these fragments via intersystem crossing. The high-resolution spectra of HCN and CO are analyzed to determine the ro-vibrational energy deposition of 81 ± 7 and 32 ± 3 kJ∕mol, respectively. With the aid of ab initio calculations, a two-body dissociation on the energetic ground state is favored leading to HCN + CH(2)CO, in which the CH(2)CO moiety may further undergo secondary dissociation to release CO. The production of CO(2) in the reaction with O(2) confirms existence of CH(2) and a secondary reaction product of CO. The HNC fragment is identified but cannot be assigned, as restricted to a poor signal-to-noise ratio. Because of insufficient excitation energy at 308 nm, the CN and CH(3) fragments that dominate the dissociation products at 193 nm are not detected.     

Photolysis of oxalyl chloride (ClCO)2 at 193 nm: emission of CO(v<or=6, J<or=60) detected with time-resolved Fourier-transform spectroscopy

Upon photolysis of oxalyl chloride at 193 nm, time-resolved and rotationally resolved emission of CO(v相似文献   

Excited state dynamics of liquid water: insight from the dissociation reaction following two-photon excitation

The authors use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The primary decay mechanism changes from dissociation at an excitation energy of 8.3 eV to ionization at 12.4 eV. The two channels occur with similar yield for an excitation energy of 9.3 eV. For the lowest excitation energy, the transient absorption at 267 nm probes the geminate recombination kinetics of the H and OH fragments, providing a window on the dissociation dynamics. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+/-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies.     

Photodissociation of the BrO radical using velocity map ion imaging: excited state dynamics and accurate D0(0)(BrO) evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-281.5 nm) and below (355 nm) the A (2)Pi(3/2) state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O((1)D(2)) threshold the contribution of the direct excitation to states other than the A (2)Pi(3/2) state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O((1)D(2)) threshold provides an accurate and direct determination of the A (2)Pi(3/2) state dissociation threshold of 35418+/-35 cm(-1), leading to a ground state bond energy of D(0)(0)(BrO)=55.9+/-0.1 kcal/mol.     

Detection of OH Radical in the Photodissociation of p-Aminobenzoic Acid at 266 nm

Photodissociation of p-aminobenzoic acid at 266 nm was investigated by probing the nascent OH photoproduct employing the laser-induced fluorescence technique. It was found that the nascent OH radical was vibrationally cold and its rotational state distribution conformed to be a Boltzmann behavior, characterized by a rotational temperature of 1040±110 K. The rotational energy of OH was determined to be 8.78±0.84 kJ/mol. Between the two spinorbit states of OH, ^2Ⅱ3/2 and ^2Ⅱ1/2, the former was found to be preferentially populated. The distribution of the II（A＇） state for the A-doublet was dominant. Finally, a probable mechanism for the formation of OH produced from the photodissociation of p-aminobenzoic acid is discussed.     

Formation of hydroxyl radical from the photolysis of salicylic acid

Photodissociation dynamics of salicylic acid (SA) in the gas phase at different photolysis wavelengths (266, 315-317 nm) is investigated by probing the nascent OH photoproduct employing the single-photon laser-induced fluorescence (LIF) technique. At all the photolysis wavelengths it is found that the nascent OH radicals are produced mostly in a vibrationally ground state (υ' = 0) and have similar rotational state distributions. The two spin-orbit and Λ-doublet states of the OH fragment formed in the dissociation are measured to have a nonstatistical distribution at each photolysis wavelength. The LIF signal of the OH could be observed upon photolysis at 317 nm but not at 317.5 nm. The threshold of OH formation from SA photodissociation is estimated to be 98.2 ± 0.9 kcal/mol. The effect of the phenolic OH group on the dissociation of SA is discussed.